Where are located custom themes?
Windows's Blender preferences, scripts, and themes are located at:

C:\Users\&lt;your username&gt;\AppData\Roaming\Blender Foundation\Blender\&lt;version number: ex. 2.78&gt;\


The theme folder is located at:

C:\Users\&lt;your username&gt;\AppData\Roaming\Blender Foundation\Blender\&lt;version number: ex. 2.78&gt;\scripts\presets\interface_theme


For example,

C:\Users\user1\AppData\Roaming\Blender Foundation\Blender\2.78\scripts\presets\interface_theme